1. Field of the Invention
The present invention relates generally to electrical wiring devices for use in an electrical distribution system, and particularly to protective electrical wiring devices.
2. Technical Background
Protective devices are used to protect personnel or property from fault conditions that arise in AC electrical distribution systems. The ground fault circuit interrupter (GFCI) is a type of protective device that is configured to detect ground leakage currents that exceeds a predetermined amount. Upon detection of excessive fault currents, the device is further configured to trip a set of interrupting contacts to protect human beings from serious electrical shock or even electrocution, or prevent the fault condition from damaging property.
The arc fault circuit interrupter (AFCI) is another class of protective devices that is configured to detect and interrupt electrical arcing. Electrical arcing is a fire hazard because it can ignite combustible materials (such as those used to construct residential or commercial structures).
Protective devices are typically found in at least three locations within a structure. For example, protective devices may be installed within the electrical wiring panel; in this instance, they are often combined with circuit breakers. Protective devices may be installed within device boxes. In this configuration they are classified as “wiring devices,” and often include receptacle outlets. In the third example, protective devices may be configured as portable devices. In this configuration, the portable protection device may be associated with load equipment, or with AC attachment plugs.
There are combination devices currently available that include both GFCI and AFCI protection. Moreover, protective wiring devices are often combined with other wiring device features. For example, GFCIs or AFCIs may include switches, dimmers, night lights, occupancy switches or receptacles.
All products that rely on electronic components and mechanical parts eventually fail over time. Protective devices are no different. Early on in the development of protective devices (such as GFCIs), a manually operable test button was provided so that the user could periodically test the device to determine if it was still operable. Moreover, the device usually came with instructions that outlined the manual test procedure. Unfortunately, users routinely ignored those instructions and many devices were installed but no longer functioning. In response to this reality, designers began to include self-testing circuitry that automatically tested the device and determined if the device was operable and protective. Briefly then, the self-test feature relieves the user from having to test the device.
An important self-test design consideration relates to the prevention of nuisance tripping. In other words, a self-test (i.e., an automatic test) should not cause the device to trip if a fault or an end-of-life condition is not extant. In one approach that has been considered, nuisance tripping is avoided by taking the protective device off line while the test is being executed. This gives the needed time to perform the test without nuisance tripping risk but unfortunately the device is unresponsive to real fault conditions that could be happening during a recurring test period.
In another approach to automated self-testing that has been considered, the protective device continues to operate (i.e., it is not taken off line); and self-testing is performed during “idle periods.” For example, a protective device may be configured to monitor the electrical distribution system for fault conditions during a predetermined AC half cycle polarity (e.g. the positive half cycle) and use the “idle” opposite polarity (e.g., the negative half cycle) for automated self-testing. Thus, the device performs both protection and self-testing without being in an offline state. In one drawback to this approach, the auto-testing process extended into the next half cycle and nuisance tripping was found to occur. Another drawback to this approach relates to the need to operate a power supply during both half-cycles of the AC line cycle. Specifically, this might require a full wave bridge power supply instead of a half-wave power supply. The full-wave power supplies often add cost and complexity.
In yet another approach that has been considered, the self-test procedure divides the auto-test into multiple test portions; wherein each test portion tests a separate portion of the protective device in a different AC cycle. While each test portion avoids nuisance tripping by taking less than a half cycle to perform, this method also has drawbacks. For example, while this process avoids nuisance tripping, the self-test procedure does not test the way the protective device truly operates when it senses and detects an authentic fault condition. Another drawback to this approach relates to the fact that, when taken together, the multiple test portions fail to test every part of the protective device. One reason for this outcome is that the device is configured to operate in a sequential manner, and when the testing is not performed sequentially, there are certain parts of the device that “are hard to reach” by separate and discrete tests (in fact, these circuits are typically not tested). Another drawback to this approach relates to the electronic components that are getting tested separately; while these parts are subject to some testing, the test does not subject them to the conditions they experience during a true fault condition. For example, in one device on the market, even though the self-test only charges a certain capacitor (in the trigger circuit) to a fraction of the voltage it must attain during the true fault condition, the test assumes that the capacitor is working properly.
Another problem that arises relates to miswiring the device. Miswire refers to a condition wherein the device installer connects the AC supply voltage to the feed-through load terminals (which are intended to be connected to downstream receptacles) instead of the line terminals of the device. If the miswired condition is not detected, the load terminals provided by the receptacles on the face of the electrical wiring device can be unprotected in the presence of a fault condition.
What is needed is a protective device that includes an automated self-testing procedure that addresses the needs outlined above in a cost effective manner. Specifically, what is needed is a self-testing protective device that performs self-testing while avoiding nuisance tripping. What is also needed is an automatic self-test that checks for a miswire condition. An automated self-testing procedure that includes multiple test portions (performed over multiple AC cycles) is also needed; a test of this type would provide for a way that tests every part of the fault detection apparatus in a manner consistent with the operating parameters of the protective device.